1. Field of the Invention
The present invention relates to a printed circuit board, a communication apparatus, and a data storage apparatus, and particularly to a printed circuit board suitable for forming differential signal paths thereon, and a communication apparatus and a data storage apparatus using the printed circuit board.
2. Description of the Related Art
As a method of transmitting a signal between LSIs, there is known a serial transmission in which information is transmitted and received by connecting the LSIs to each other with the use of differential transmission lines composed of two signal transmission lines. The transmission lines which are connected between the LSIs and which are arranged on a printed circuit board for serial transmission are normally arranged through holes that are called penetrating through holes or via holes. The number of via holes used for the signal transmission lines increases as the number of signal transmission lines for connecting between the LSIs increases.
The transmission lines arranged on different layers within the printed circuit board are mutually connected in an electrical manner through the via holes. However, in the case of high-speed signal transmission more than 1 GHz between the LSIs, the via hole itself causes the distortion of its waveform. The transmission lines on the printed circuit board configure transmission line paths called a stripline or a microstripline, and the characteristic impedance thereof can be easily kept constant. Irrespective of the fact, the distortion is caused due to difficulty of adjusting the impedance in the via holes to that of the transmission lines.
For the differential signal paths, a design policy called differential impedance is used in signal transmission. The characteristic impedance of wiring of a single-ended transmission line is defined by a ratio of propagation voltage to propagation current of a signal that is transmitted from the LSI. The characteristic impedance of the differential transmission lines is in relation to a propagation mode defined by directions of signals flowing in two transmission lines. In the case where polarities of signals between the two transmission lines are opposite to each other, it is called an odd mode. On the other hand, in the case where the signals are in phase, it is called an even mode. A differential impedance (Zdiff) and a common impedance (Zcom) with which the two transmission lines are terminated have relations of the following formulae.Zdiff=2·Zodd  (1)Zcom=Zeven/2  (2)
where the symbol · represents multiplication, Zodd represents the mode impedance of the odd mode, and Zeven represents the mode impedance of the even mode.
Since the polarities of signals in the differential transmission lines are always opposite to each other, if the two transmission lines are terminated with the differential impedance (Zdiff), the perfect matching condition of no reflection can be realized. For example, when two uncoupled transmission lines each having 50 Ω are prepared for differential signal transmission, the perfect matching condition that is in a no-reflection state is 100 Ω of the differential impedance (Zdiff) which is twice the characteristic impedance of a single-ended transmission line. In the case of coupled transmission lines, coupling of single-ended transmission lines each having a characteristic impedance of 55 Ω to 60 Ω allows the differential impedance to be lowered to 100 Ω.
As similar to the characteristic impedance of the single-ended transmission line, the differential impedance is determined roughly by the cross sectional shapes of the transmission lines. Specifically, the shapes imply a line width, a line thickness, the thickness of a dielectric layer, a relative permittivity of a dielectric material, and spacing between the transmission lines.
The differential impedance (Zdiff) has been mainly utilized for design of differential signals. This is because, if the differential impedance is constant throughout the system of the transmission line paths, it is considered as no reflection. Further, the differential signals have a characteristic of the common mode impedance (Zcom) of the in-phase propagation mode, and a wiring mode in which the differential impedance is constant but the common mode impedance is different is conceivable.
In the transmission pathways, since the cross sectional shapes of the transmission lines within the printed circuit board can be kept constant, the differential impedance can be constant. On the other hand, voltage and current of signals propagating through via holes are different from each other in movement. Although signal voltage propagates in the vertical direction with an assumption that the direction in which signals flow in the via holes is called vertical, the propagation voltage reaches throughout the via holes whereas the current flows only in a portion where the transmission lines are connected. For example, in the case of a via hole between the transmission lines connecting the uppermost layer and the lowermost layer in a multilayer printed circuit board, signal current flows throughout the via hole. However, in the case of a via hole connecting the uppermost layer and the second layer from the top (which is not the lowermost layer), propagation voltage reaches throughout the via hole. On the other hand, signal current does not flow throughout the via hole but between the uppermost layer and the second layer. This is because the latter requires less potential energy.
The pathways of voltage and current flowing in the via hole are different from each other. Accordingly, the impedance of the via hole changes depending on a wiring layer from which the transmission line is pulled out even if the via hole has a columnar shape, or clearances of a ground surrounding the via hole have the same structure. The impedance of the via hole does not match the characteristic impedance of the pulled-out transmission lines, so that reflection noise is generated due to impedance mismatch immediately before and after signals pass through the via hole. This impedance mismatch causes the distortion of a signal waveform due to the via hole.
A portion where current does not flow is called an open-stub. The open means disconnection, and the stub means a branch line. A portion except a pulled-out portion in the transmission line serves as a branch line, and an end of the transmission line except the pulled-out portion is an open end which is not connected, thus the portion is called open-stub.
As a method of solving the line distortion due to the via hole, there is a method of chipping off the open-stub portion of the via hole arranged in the printed circuit board. The method is called a back drill. The back drill is described in Japanese Patent Application Laid-Open No. 2004-235629 and “Design advances in PCB/backplane interconnects for the propagation of high speed signals”, F. Gisin, et al., pp. 184-191 vol. 1, Digital Object Identifier 10.1109/TELSIKS. 2003.
However, the back drill is applied to each via hole including the open-stub, which increases the steps of preparing printed circuit boards and causes a rise in cost. Especially in a printed circuit board such as a mother board for a backplane bus, on which plural connectors are mounted, the number of via holes reaches from the thousands to the tens of thousands, which increases the cost of the back drill, and results in an increase of the cost of the apparatus.
The backplane bus is disclosed in U.S. Pat. No. 6,812,803.